Both the RUC (now Police Service of Northern Ireland) and Garda Special Branches had informants working inside or on the periphery of the republican terrorist group responsible for the Omagh atrocity.
Up to the early summer of 1998, the Garda had a run of successes against the group, intercepting car bombs and explosives on their way to attacks in Northern Ireland and Britain.
The RUC, too, is understood to have had at least one well-placed informant working within the "Real IRA". It is suspected this man is from Newry, Co Down, and that he was quite close to the leadership of the group.
According to Garda sources there was information that the "Real IRA" was preparing explosives near Kilcock, Co Meath, in the weeks before the Omagh bombing and that sufficient explosives for two bombs - more than 1,000 lbs of fertiliser and sugar mix - were transported in a stolen car to the Border.
It is understood that this car was under surveillance at some point but it was lost at or near the Border crossing near Cullaville in south Armagh.
Half of the explosives were then taken to a test-firing site in the Cooley Mountains and detonated, just over a week before the attack.
There were other clear indications that the "Real IRA" was planning a major attack and that the likely target would be a town centre. Two weeks to the day before Omagh the "Real IRA" had come close to causing carnage in its attack on Banbridge, Co Down.
Again the target was a busy (mainly Protestant) town and a Saturday afternoon at the busiest point of the week. There was also an indistinct warning and the RUC and British army were barely able to clear the centre before the explosion. Some 35 people were injured when the 500-lb bomb - again similar to the Omagh bomb - went off.
There had also been attacks earlier in the year on town centres in Portadown (February 23rd), Markethill, Moira, Co Down (February 20th) and at Banbridge again on January 6th. The first "Real IRA" bombing was at Markethill, Co Armagh on September 16th the previous year.
Despite these attacks, far more attacks were stopped and it became clear that both the Garda and RUC had intelligence from sources inside or close to the bombers.
According to Garda sources the main sources of information about the "Real IRA" at the time came from criminals on the verge of the organisation who were supplying stolen cars for bombings. This gang was infiltrated and the movements of cars tracked.
The cars were being taken to the premises in Kilcock where explosives were being manufactured by a man with long-standing connections to the Provisional IRA who appears to have joined the new group.
The Garda sources have stated that while there was, at times, good intelligence about the activities of the "Real IRA" in the Republic, this information dried up as soon as they crossed the Border.
Inside south Armagh, where the Omagh bomb was made, the main intelligence source would seem to have been the RUC's informant or informants.
It had not been known until yesterday's leaked information from the Northern Ireland Police Ombudsman's report - if this proves to be accurate - that there was prior information about Omagh being a target.
It has been known in the past that Special Branch officers have allowed attacks to take place in order to protect their informants from being detected.